Cold cathode ionization vacuum gauge

ABSTRACT

A cold cathode ionization vacuum gauge includes an extended anode electrode and a cathode electrode surrounding the anode electrode along its length and forming a discharge space between the anode electrode and the cathode electrode. The vacuum gauge further includes an electrically conductive guard ring electrode interposed between the cathode electrode and the anode electrode about a base of the anode electrode to collect leakage electrical current, and a discharge starter device disposed over and electrically connected with the guard ring electrode, the starter device having a plurality of tips directed toward the anode and forming a gap between the tips and the anode.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/500,820, filed on Sep. 29, 2014, which corresponds toInternational Application No. PCT/US2014/058088, filed on Sep. 29, 2014,which claims the benefit of U.S. Provisional Application No. 61/884,797,filed on Sep. 30, 2013. The entire teachings of the above applicationsare incorporated herein by reference.

BACKGROUND OF THE INVENTION

Cold cathode ionization vacuum gauges are well known. Three commonlyknown cold cathode ionization vacuum gauges include normal (noninverted)magnetron type gauges, inverted magnetron type gauges, and Philips (orPenning) gauges. All of these types of gauges have a pair of electrodes(i.e., an anode and a cathode) in an evacuated non-magnetic envelopewhich is connected to the vacuum to be measured. A high DC voltagepotential difference is applied between the anode electrode and thecathode electrode to cause a discharge current to flow therebetween. Amagnetic field is applied along the axis of the electrodes in order tohelp maintain the discharge current at an equilibrium value which is afunction of pressure.

Accordingly, a cold cathode ionization vacuum gauge (CCIVG) provides anindirect measurement of vacuum system total pressure by first ionizinggas molecules and atoms inside its vacuum gauge envelope and thenmeasuring the resulting ion current. The measured ion current isdirectly related to the gas density and gas total pressure inside thegauge envelope, i.e., as the pressure inside the vacuum systemdecreases, the measured ion current decreases. Gas specific calibrationcurves provide the ability to calculate total pressures based on ioncurrent measurements.

A significant difference between a CCIVG and a hot cathode ionizationvacuum gauge (HCIVG) is the lack of a hot filament to establish an ioncurrent in a CCIVG. The lack of a hot filament simplifies theconstruction and operation of the CCIVG and improves its reliability, asthere is no risk of filament burn-out by sudden or accidental exposureof the gauge to a high pressure. The lack of a hot filament, on theother hand, complicates gauge monitoring as there is no independentelectron current to be measured and controlled, unlike in a HCIVG wherethe electron emission current is monitored and used to assure thevalidity of the ion current measurements. In other words, as the CCIVGstarts to lose sensitivity, both the electron and ion currents decreaseover time; however, since the user does not have direct access toelectron current (in contrast to a hot cathode gauge), there is no wayto know whether a drop in ion current is due to a reduction in electroncurrent in the discharge or a reduction in process pressure.

In cold cathode ionization vacuum gauges of the inverted magnetron type,it is possible for a small leakage current to flow directly from theanode to the cathode via the internal surfaces of the gauge, and it isknown that the presence of a so-called “guard ring” can collect thisleakage current and thereby prevent it from reaching the cathodeelectrode and being detected by the gauge itself. To perform thisfunction, the guard ring is electrically isolated from the cathodeelectrode and normally held at a small positive voltage potentialdifference relative to the cathode electrode.

Another aspect of cold cathode ionization vacuum gauges is that, as thepressure decreases, the gauge can take longer and longer times to startthe discharge that is used to provide the ion current that is used tomeasure pressure. Many designs have been used to seed electrons into thedischarge volume to trigger the avalanche process that is responsiblefor building up the discharge.

Nevertheless, there continues to be a need for improved cold cathodeionization vacuum gauges that minimize or eliminate the problemsdescribed above.

SUMMARY OF THE INVENTION

The invention is generally directed to a cold cathode ionization vacuumgauge and methods of operation thereof. The cold cathode ionizationvacuum gauge includes an extended anode electrode and a cathodeelectrode surrounding the anode electrode along its length and forming adischarge space between the anode electrode and the cathode electrode.The vacuum gauge further includes an electrically conductive guard ringelectrode interposed between the cathode electrode and the anodeelectrode about a base of the anode electrode to collect leakageelectrical current, and a discharge starter device disposed over andelectrically connected with the guard ring electrode, the starter devicehaving a plurality of tips directed toward the anode and forming a gapbetween the tips and the anode. The plurality of tips can, for example,be numbered in a range of 2 tips to 8 tips, such as in a range of 5 tipsto 7 tips, or 6 tips. The gap between the tips and the anode can be in arange of between about 500 μm and about 2500 μm. The starter device canbe made of stainless steel, tungsten, or other metal or conductivematerial. The voltage potential difference between the starter deviceand the anode, during operation of the cold cathode ionization vacuumgauge, can be in a range of about 0.4 kilovolts (kV) to about 6 kV, suchas about 3.5 kV. Optionally, the voltage potential difference betweenthe starter device and the anode can be configured to be increased fromabout 3.5 kV to about 5 kV during startup of the gauge.

The cold cathode ionization vacuum gauge can include a removable anodesleeve. The removable anode sleeve can be a thin walled tube on theanode post that provides an electrical connection to the anode post andshields the anode post surface from contaminants. The anode sleeve maybe held in place on the anode post with a friction fit. The cold cathodeionization vacuum gauge can also include a removable cathode.

The cathode electrode of the cold cathode ionization vacuum gauge canhave an opening to receive gas from a monitored chamber, and the vacuumgauge can further include a baffle across the opening of the cathode tolimit flow of sputtered material to the chamber. The baffle can beconfigured as a plurality of slots or holes disposed at an angle withrespect to the anode. The angle can be in a range of about 0 degrees toabout 60 degrees, such as about 45 degrees. Alternatively oradditionally, the baffle can be composed of at least two partitions,each partition having at least one aperture, the apertures located outof a line of sight between the chamber and the cathode.

The cathode electrode of the cold cathode ionization vacuum gauge canhave one end connected to a flange, and a magnetic coupler can beconnected to the flange. A magnet assembly is configured to be slidablymounted over the cathode and magnetically coupled to the magneticcoupler.

The vacuum gauge can further include an electronics module configured tobe directly coupled to the vacuum gauge with an interface complementaryto the vacuum gauge, the module housing electronics adapted to operateand read the vacuum gauge. The electronics module can further include aninterlockconfigured to lock to the vacuum gauge. The interlock of theelectronics module can further include a gauge detector configured todetect the presence of the vacuum gauge and provide a correspondinggauge detect signal. The gauge detect signal can indicate whether or notthe vacuum gauge is properly locked to the electronics module. In someaspects, the electronics module can further include a magnet on a frontface of the electronics module adapted to hold the vacuum gauge in placeuntil the interlock is engaged.

The vacuum gauge can further include a cable between the electronicsmodule and the vacuum gauge with the vacuum gauge and electronics moduledisplaced from each other, the cable having a first end and a secondend, the first end being configured to imitate physical mating surfacesof the vacuum gauge to mate to the electronics module, and the secondend being configured to imitate physical mating surfaces of theelectronics module to mate to the vacuum gauge.

Methods of operating a cold cathode ionization vacuum gauge includesetting a voltage potential difference to form an electrical dischargebetween the anode electrode and the cathode electrode, measuring adischarge impedance between the anode electrode and the cathodeelectrode, and deriving a pressure reading therefrom. A method includesswitching the voltage potential difference between a high voltagesetting and a low voltage setting. The switch is made at a pressure thatis lower than that of a high voltage measurement anomaly and at apressure that is higher than that of a low voltage measurement anomaly.The high voltage measurement anomaly and the low voltage measurementanomaly can be discharge current anomalies. The high voltage setting canbe in a range of about 3.5 kV to about 6 kV, and the low voltage settingcan be in a range of about 2 kV to about 3 kV.

Another method of operating a cold cathode ionization vacuum gaugeincludes measuring a leakage electrical current between an electricallyconductive guard ring electrode interposed between the cathode electrodeand the anode electrode about a base of the anode electrode, andtriggering a gauge maintenance alarm if the pressure reading is lessthan an oscillatory discharge current pressure level and the leakageelectrical current is greater than a maximum allowable leakage currentlimit. The oscillatory discharge current pressure level can be about5×10⁻⁶ Torr. The maximum allowable leakage current limit can be about 1μA.

Yet another method includes measuring a discharge current between theanode electrode and the cathode electrode, and deriving a pressurereading therefrom, recording the discharge current as a function oftime, and integrating the discharge current over time to obtain apressure dose for the vacuum gauge. The method can further includerecording and integrating a gas factor and/or an ion energy factor as afunction of time. Additionally, the method can include determining aremaining service life for the vacuum gauge based on the pressure dose.

This invention has many advantages, including long term stability due toseparate measurement of discharge current and leakage current,reproducible and rapid startup due to the starter device, and avoidanceof measurement anomalies during operation of the cold cathode ionizationvacuum gauge.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1A is a cross-sectional illustration of a cold cathode ionizationvacuum gauge.

FIG. 1B is an exploded cross-sectional view of the component parts of acold cathode ionization vacuum gauge.

FIG. 1C is a cross-sectional view of the component parts of a SHVfeedthrough of a cold cathode ionization vacuum gauge.

FIG. 1D is an illustration of an optional anode sleeve.

FIG. 1E is a cross-sectional illustration of a cold cathode ionizationvacuum gauge with an optional anode sleeve.

FIG. 1F is an exploded cross-sectional view of the component parts of acold cathode ionization vacuum gauge with an optional anode sleeve.

FIG. 2A is a cross-sectional perspective view of the starter device fora cold cathode ionization vacuum gauge.

FIG. 2B is a perspective view of the starter device for a cold cathodeionization vacuum gauge.

FIG. 2C is a bottom view of the starter device for a cold cathodeionization vacuum gauge.

FIG. 3 is a perspective view of electrical contacts to a vacuum gaugeinside an electronics module for a cold cathode ionization vacuum gauge.

FIG. 4 is a schematic illustration of the discharge current and guardring leakage current measurement circuits of a cold cathode ionizationvacuum gauge.

FIG. 5 is a graph of ln(I/E²) as a function of 1/E demonstrating the fitof the data for a cold cathode ionization vacuum gauge to the FowlerNordheim equation.

FIG. 6 is a graph of the starting probability as a function of time atthree different pressures for a cold cathode ionization vacuum gauge.

FIGS. 7A-7C are cross-sectional illustrations of three baffle designsfor a cold cathode ionization vacuum gauge.

FIG. 8 is a cross-sectional perspective illustration of a baffle designhaving two partitions for a cold cathode ionization vacuum gauge.

FIG. 9A is a perspective view of an electronics module and cold cathodeionization vacuum gauge prior to assembly.

FIG. 9B is a perspective illustration of a latch interlock for theelectronics module shown in FIG. 9A.

FIG. 10 is a perspective illustration of a cold cathode ionizationvacuum gauge.

FIGS. 11A-11C are cross-sectional illustrations of a cable connectingthe electronics module to the cold cathode ionization vacuum gauge.

FIG. 11D is a perspective view of an electronics module and cable endthat imitates physical mating surfaces of a cold cathode ionizationvacuum gauge.

FIG. 11E is an exploded view of the component parts of the cable endshown in FIG. 11D.

FIG. 12 is a graph of discharge impedance (ohm) as a function ofpressure (Torr) showing an example of a low operating voltagediscontinuity (LOV: Discontinuity), and an example of a high operatingvoltage discontinuity (HOV: Discontinuity) for a cold cathode ionizationvacuum gauge.

FIG. 13 is a graph of discharge current (A) as a function of pressure(Torr) for a cold cathode ionization vacuum gauge.

FIG. 14 is a graph of anode voltage (volts) as a function of pressure(Torr) for a cold cathode ionization vacuum gauge.

FIG. 15 is a graph of discharge impedance (ohm) as a function ofpressure (Torr) for a cold cathode ionization vacuum gauge.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

The cold cathode ionization vacuum gauge described herein relies on theinverted magnetron principle. The gauge is of cylindrical symmetry. Alarge voltage potential difference (i.e., radial electric field) betweenthe anode pin (located at the axis) and the cathode cylindrical envelopeprovides energy to the electrons for the ionization events to occur. Acrossed axial magnetic field provides the electron trajectory pathlength required to maintain a discharge inside the envelope. Thedischarge current is the measured quantity that is proportional to thepressure in the system.

The discharge is established through an avalanche ionization processthat generally starts with a single electron being released into theionization volume of the gauge. The process responsible for releasing anelectron can include a field emission event or a cosmic ray ionizationprocess. The avalanche process relies on the long path length for theelectron trajectories that leads to many ionization processes perelectron. Each ionization process releases an ion as well as anadditional electron that is added into the discharge. As the ionscollide with the cathode internal walls, additional electrons are alsoreleased into the discharge, thereby contributing to the total charge.The electrical discharge consisting of ions and electrons reaches an iondensity that is proportional to the pressure in the system.

The cold cathode ionization vacuum gauge described herein relies on thedouble inverted magnetron principle introduced by Drubetsky in 1995. SeeU.S. Pat. No. 5,568,053. The double inverted magnetron design, shown inFIG. 1A, includes two magnets held together in a magnet assembly, thetwo magnets having their magnetic poles opposed to one another. Thedouble inverted magnetron features some of the largest magnetic fields,and, as a result, provides the largest gauge sensitivities available.Large gauge sensitivities are required to be able to read reliablepressures at UHV levels (i.e., pressures less than about 10⁻⁹ Torr).

Accordingly, in one aspect, shown in FIGS. 1A, 1B, and 1C, a coldcathode ionization vacuum gauge 100 includes a floating SHV (safe highvoltage) stainless steel cylindrically symmetric feedthrough 101, shownin FIG. 1C separately from the rest of the vacuum gauge 100, thatincludes a guard ring BNC style connection 102 that provides electricalconnection to a guard ring electrode 140 described below. Inside theguard ring connection 102, an anode guard ring insulator 106 provideselectrical insulation around an anode connection 110 a to an extendedanode electrode 110. The guard ring electrode 140 is connected to astarter device 150, which is described below. The guard ring connection102 is connected by a cathode-guard ring insulator 103 to a weld surface104, which is seam welded to a monolithic flange assembly 105. As shownin FIGS. 1A and 1B, the monolithic flange assembly 105 includes outerflange 105 a and inner flange 105 b. The inner flange 105 b encloses acathode electrode 120 surrounding the anode electrode 110 along itslength and forming a discharge space 130 between the anode electrode 110and the cathode electrode 120. A baffle, described below and shown inFIGS. 1A and 1B as two partitions 170 and 180 having apertures 175 and185, respectively, is connected to the cathode electrode 120. As shownin FIG. 1B, the cathode electrode 120 and baffle partitions 170 and 180are removable from inner flange 105 b, enabling refurbishing of thevacuum gauge 100. The cathode electrode 120 and baffle partitions 170and 180 are retained inside inner flange 105 b by a snap ring 107. Arefurbishing kit for vacuum gauge 100 can include a replacement cathode120 and baffle partitions 170 and 180, and, optionally, an anode sleeve110 b, shown in FIGS. 1D, 1E and 1F, that covers the surface of anodeelectrode 110, thereby replacing or covering the surfaces that aresubject to sputtering or deposition during operation of the vacuum gauge100. To aid in evacuation of the vacuum gauge 100 after replacement ofthe cathode 120, a small gap 108, shown in FIG. 1A, is provided betweenthe cathode 120 and the inner flange 105 b. The gap 108 terminates atthe end of the cathode 120 close to the guard ring electrode 140, inorder to provide proper alignment of the cathode 120 with the guardelectrode 140 and anode electrode 110.

The removable and replaceable anode sleeve 110 b may be a thin walledtube that slides onto the anode post 110, providing electricalconnection to the anode post and shielding the post surface fromcontamination build-up. The anode sleeve 110 b may simply be held inplace with a friction fit. The anode sleeve 110 b may also include ahole 110 c on the top end that can be used to hook the sleeve in orderto pull it out of the gauge structure. The bottom end of the anodesleeve 110 b may include a flare 110 d. The anode sleeve flare 110 d caninhibit deposits from falling into the starter device 150.

Once a gauge shows signs of contamination, it should be possible toquickly and easily clean the contaminated areas of the gauge and restoreit to a fully functional condition. The anode in a CCIVG is expected tobuild a layer of hydrocarbon or silicone contaminants very quickly in avacuum system. A removable anode sleeve 110 b can be used in CCIVGs toprotect and shield the anode post from contamination build up during theoperation of the gauge. Used in combination with the removable cathode120, it provides a fast and easy method of field servicing a CCIVG and aCCIVG user is able to restore the entire gauge in a matter of a fewminutes. This method of servicing a CCIVG has several advantages overprevious methods. Using a removable and replaceable anode sleeve 110 band cathode eliminates the need to use abrasive materials to clean theanode post and cathode once they are contaminated. Abrasive cleaners mayabrade away too much material from the internal surfaces of the gaugeand significantly impact their mechanical and chemical characteristics.With a removable sleeve and cathode, the contaminated surfaces can beeasily removed and replaced with a clean set. The contaminated cathodecage and the anode sleeve do not have to be cleaned by means of abrasivetreatments. In most cases, the cleaning can be done in an ultrasoniccleaner with the help of proper chemicals. One way to eliminatehydrocarbon build up on the anode post is to use alkaline cleaners suchas a concentrated solution of NaOH followed by a rinsable cleaningsolution. Using a removable anode sleeve and cathode reduces thepossibility of affecting the concentric alignment between the anode andthe cathode because the gauge does not need to be fully disassembled.The maintenance cost is minimal. A user with access to an extra anodesleeve/cathode combination can quickly do a swap each time maintenanceis required.

As discussed above, a crossed axial magnetic field provides the electrontrajectory path length required to maintain a discharge inside thedischarge space 130. The magnetic field is created by magnet assembly115, shown in FIGS. 1A and 1B, which includes two Samarium Cobalt (SmCo)magnets 115 a and 115 b that are glued together one on top of the otherwith their magnetic poles opposite to one another in a double inverted(DI) configuration, and enclosed by a stainless steel cylinder 112 whoseends 112 a and 112 b are swaged closed, thereby confining the magnets115 a and 115 b, which would otherwise repel each other due to theopposite pole configuration. The magnet assembly 115 includes aferromagnetic spacer 114, which connects the magnet assembly 115 to theinner flange 105 b by the aid of a locating pin 117 pressed into theinner flange 105 b, and a magnetic coupler 116 (shown as a ferromagneticring 116 in FIG. 1B). The magnetic coupler 116 is spot welded to theinner flange 105 b. The magnet assembly 115 is slidably connected to theinner flange 105 b at a location with respect to cathode electrode 120that locates the electrical discharge inside discharge space 130. Themagnet assembly 115 can also include an aluminum (or other non-magneticmaterial) spacer 113 at the end of the magnet assembly closest to theguard ring 140 to adjust the location of the electrical discharge awayfrom the guard ring 140.

The electrically conductive guard ring electrode 140 is interposedbetween the cathode electrode 120 and the anode electrode 110 about abase of the anode electrode 110 to collect leakage electrical currentthat would otherwise tend to flow between the anode electrode 110 andthe cathode electrode 120 if electrically conductive deposits accumulateover time on surfaces of the cathode-guard ring insulator 103 exposed tothe discharge space 130 during operation of the vacuum gauge 100.

A discharge starter device 150 is disposed over and electricallyconnected with the guard ring electrode 140. As shown in FIG. 2A, thestarter device 150 has a plurality of tips 260 (3 tips are shown in thecross-section cylindrically symmetrical view shown in FIG. 2A) directedtoward the anode 110 and forming a gap between the tips 260 and theanode 110. The plurality of tips 260 can be numbered in a range of 2tips to 8 tips, such as in a range of 5 tips to 7 tips, or 6 tips, asshown in FIG. 2C, where another view of starter device 150 shows 6 tips260, of which one tip 260 is hidden behind anode electrode 110. The tipscan be designed in various patterns, such as star bursts. See U.S. Pat.No. 8,120,366 for examples of starter tip shapes. The gap between thetips and the anode can be in a range of between about 500 μm and about2500 μm. The gap is configured such that the field emission currentduring normal operation is in a range of about 1 pA to about 10 pA whena voltage potential difference between the starter device 150 and theanode 110 is established. The field emission current amplitude isdependent on several parameters, such as the voltage potentialdifference, the size of the gap, the number of points on the starterdevice, and the type of material that the starter device is made of. Thestarter device 150 can be made of stainless steel, tungsten, or othermetal or conductive material. The voltage potential difference betweenthe starter device and the anode, during operation of the cold cathodeionization vacuum gauge, can be in a range of about 0.4 kV to about 6kV, such as about 3.5 kV. This voltage potential difference produceselectrons by field emission from the sharp tips 260 to the anode,thereby seeding some electrons into the discharge volume 130 to triggerthe avalanche process that is responsible for building up the discharge.Optionally, the voltage potential difference between the starter deviceand the anode can be configured to be increased from about 3.5 kV toabout 5 kV during startup of the gauge, in order to increase the fieldemission current by increasing the high voltage supply bias to the anodeelectrode momentarily, until a discharge is detected by a suddenincrease in the discharge current.

During operation of the vacuum gauge 100, electrical contacts to theanode electrode 110 and guard ring electrode 140 are made by the anodeconnection 110 a and the guard ring connection 102, respectively.Electrical contact to the cathode electrode 120, which is grounded, ismade either by a spring clip 310 shown in FIG. 3, which is locatedinside an electronics module described below, the spring clip 310contacting the inner flange 105 b, or by a wire attached to a cable, asdescribed below, the wire being attached to any part of the outer flange105 a (e.g., attached to a bolt that fastens the outer flange 105 a to avacuum chamber (not shown)). As also shown in FIG. 3, contact to theguard ring connection 102 inside the electronics module is made byanother spring clip 320.

As shown in FIG. 4, an advantage of having the starter device 150disposed over and electrically connected to the guard ring 140 is thatthe guard ring current I_(GR), which is the sum of the leakage currentI_(L) and the field emission current I_(FE)(I_(GR)=I_(L)+I_(FE)), ismeasured by ammeter 470 separately from the discharge current I_(D),which is measured by ammeter 460, from which the pressure is derived bya suitable calibration curve. The field emission current I_(FE) can becalculated using the Fowler Nordheim equation

$\begin{matrix}{I_{FE} = {\frac{1.54 \times 10^{6}{A\left( {\beta \; E} \right)}^{2}}{\phi}^{({- \frac{683 \times 10^{3}\phi^{3/2}}{\beta \; E}})}}} & (1)\end{matrix}$

where E is the electric field (MV/m), Φ is the work function of thematerial (eV), β is the field enhancement factor, and A is the effectiveemitting area (m²). As shown in FIG. 5, the graph of leakage current asa function of electric field shows a good fit to Eq. 1 and thereforedemonstrates that there is field emission current from the guard ringstarter device to the anode.

Since the start times of a cold cathode ionization vacuum gauge are of astatistical nature, meaning that under the same conditions differentstart times will be measured each time, if enough samples are measured,a distribution of start times is generated. While it is difficult tospecify a start time from a histogram of start times, if a cumulativeprobability is used, which is the normalized integral of the startingtime histogram, the starting probability can be predicted at any timeand voltage at a given pressure. Starting times are measured by turningoff all sources of ions in a vacuum chamber; after a waiting period, thehigh voltage power supply (HVPS) 430 to the gauge is turned on, and boththe cathode discharge current I_(D) and the guard ring current I_(GR)are measured; when the cathode current jumps from its nominal baselineof 10ths of nanoamperes to operational values several decades larger,the start time is logged. The system is automated to repeat thiscollection method, so that start time statistics can be accumulated.

FIG. 6 shows the starting probability curve for a cold cathodeionization vacuum gauge at three different pressures, showing the effectof system pressure on start time for a cold cathode ionization vacuumgauge having a gap of about 762 μm (0.030″) in the starter device and a4 kVolt voltage potential difference between the anode electrode and thecathode electrode. Using this data, it can be said that at 1×10⁻⁸ Torrthere is a 60% chance of starting in 100 seconds, and at 1×10⁻⁶ Torr thevacuum gauge will always start in less than 20 seconds. Equation 1 showsthat the field emission current increases as the work functiondecreases. A starter device made of stainless steel alloy has a workfunction of about 4.4 eV. A metal with a lower work function would helpdecrease start time by increasing the field emission current, while ametal with a higher work function could be used to reduce the emissioncurrent and hence reduce the starting time of the CCIVGs. Variations instarter designs include varying not only the actual mechanical design,but also the material used. It is expected that varying the materialwill vary the work function and also change the field emission levels.Starting time optimization includes using the statistical plotsdescribed here for metrics of improvements, and adjusting the starterparameters including the number and shape of field emitting points, thegaps between anode and field emitting points (this could include notonly changing the starter design, but also using sleeves of differentwall thicknesses and materials in the anode to carefully adjust gaps),the voltage potential difference between the anode electrode and thestarter device, and the material of construction of the starter device.

Turning to FIGS. 7A-7C and 8, the cathode electrode of the cold cathodeionization vacuum gauge can have an opening to receive gas from amonitored chamber, and the vacuum gauge can further include a baffleacross the opening of the cathode to limit flow of sputtered material tothe chamber. The sputtered material results from erosion of the materialof the cathode 120 by energetic impact between positive ions in thedischarge volume 130 and the internal surface of the cathode 120, asshown in FIG. 7A. The baffle can be configured as a plurality of slots710, chevrons 720, or holes 730 disposed at an angle with respect to theanode 110, as shown in FIGS. 7A-7C, respectively. The angle can be in arange of about 0 degrees, as shown in FIG. 7C, to about 60 degrees, suchas about 45 degrees, as shown in FIG. 7A. Alternatively or additionally,the baffle 800 can be composed, as shown in FIG. 8, of at least twopartitions 170 and 180, each partition having at least one aperture 175in partition 170, and at least one aperture 185 (shown as a plurality ofholes 185 in FIG. 8) in partition 180, the apertures of the partitionslocated out of a line of sight between a chamber 890 (not shown indetail) and the discharge volume 130. The partition 170 facing thechamber 890 allows gas to flow in and out through apertures (e.g., holesor slots) without substantially limiting gas conductance, provides anupper electrical boundary condition for the electric field, and providesshielding against the escape of sputtered material out of the cathode120. The designs shown in FIGS. 7A-7C and 8 take advantage of thedirectionality of ejected sputtered materials to reduce the escape ofsputtered material while still supporting high gas conductance. Thesedesigns can be considered to be baffles designed to stop material fromescaping the CCIVG ionization area, but can also be considered bafflesto stop materials coming from the process chamber into the ionizationregion 130. The common feature of these designs is a short profile withhigh gas conductivity and blind to line of sight contaminants. In thatrespect, these baffles are not limited to cold cathode ionization vacuumgauges, and can be used in a variety of vacuum gauge designs.

Cold cathode ionization vacuum gauges presently available commerciallytypically have 1) a gauge connected to a controller through a cableinterconnect, or 2) a gauge connected directly to an electronics module,i.e. with no cable interconnect. The choice between the two technologiesseems to be defined by (1) the need to bake out the gauge while thegauge is operating and (2) a need to operate the electronics remotelyfrom the gauge. Most modular CCIVGs require a direct connection betweenthe gauge and the module, and tools are often required to separate thegauge from the controller. In some cases the magnet assembly is part ofthe electronics module, and in some other cases the magnets are part ofthe gauge which might require an additional tool to separate them fromthe gauge tube. CCIVGs with modular configurations generally includeo-ring or compressed glass fittings and generally do not allow extensiveand/or high temperature bakeouts. The lack of flexibility of modulardesigns limits their applicability and drives many users to morecomplicated and costlier products that include remote controllers.

The cold cathode ionization vacuum gauge described herein combines thebest features of the commercially available vacuum gauges describedabove by including both an optional cable and an electronics module. Asshown in FIG. 9A, the vacuum gauge assembly 900 further includes anelectronics module 910 configured to be directly coupled to the vacuumgauge 100 with an interface 930 complementary to the vacuum gauge 100,the module 910 housing electronics adapted to operate and read thevacuum gauge 100. The electronics module 910 can further include aninterlock lever 940 configured to lock the electronics module 910 to thevacuum gauge 100. For safety reasons, it is important that theelectronics module is properly locked to the vacuum gauge. If theelectronics module is not properly locked to the vacuum gauge, it mayexpose the user to dangerous high voltage. Therefore, the electronicsmodule can further include a gauge detector configured to detect thepresence of the vacuum gauge and provide a corresponding gauge detectsignal. The gauge detect signal can indicate whether or not the vacuumgauge is properly locked to the electronics module. In some aspects, theelectronics module can further include a magnet (not shown) on a frontface of the electronics module adapted to hold the vacuum gauge in placeuntil the interlock is engaged.

As shown in FIG. 9B, where the interlock 945 with lever 940 is shownupside down from the view shown in FIG. 9A for a better view of thecomponent parts discussed below, the interlock 945 can include a latchmade up of a ring 955 that is pressed in a plate 950 and a surface 965on a second plate 960. The two surfaces 955 and 965 lock, as shown inFIG. 10, into a groove 1010 on the vacuum gauge 100 to secure theconnection between the electronics module 910 and the vacuum gauge 100(see FIG. 9A). Turning back to FIG. 9B, the plate 960 slides againstplate 950 in key slots 970 a and 970 b against tension provided bysprings 980 a and 980 b that keep the latch 945 in the normally closedposition shown in FIG. 9B. When the interlock lever 940 is pushed in,the latch opens, the surfaces 955 and 965 forming a perfect circle, andthe vacuum gauge 100 can pass through the latch 945.

To detect the interlock of the vacuum gauge with the electronics module,the position of the plate 960 is detected. To detect the position of theplate, a finger 990 a is carried by the plate 960. When the interlock945 is not engaged, the gauge detector finger 990 a depresses a gaugedetector button shown schematically at 990 b; that is, the button ispushed up in FIG. 9B. As the interlock lever 940 is engaged and pushedin, the latch 945 starts to open and the gauge detector finger 990 amoves in the same direction as the interlock lever releasing the buttonto move down in FIG. 9B. When the vacuum gauge passes through the latch945 and the surfaces 955 and 965 lock into the groove 1010 on the vacuumgauge, the finger 990 a is held away from underneath the button 990 b.In this configuration, the gauge detector lever 990 a does not makecontact with the gauge button 990 b. A gauge detect signal can betriggered in this configuration to indicate that the vacuum gauge isproperly locked to the electronics module.

The design shown in FIG. 10 yields an all-metal gauge with no o-ringsand with the ability to remove the magnet assembly 115 without anytools, by sliding the magnet assembly 115 upwards as shown in FIG. 1B(see magnet assembly 115, which slides over cathode 120 and connects tomagnetic coupler 116, with the aid of locating pin 117 as describedabove), enabling high-temperature (e.g., 250° C.) bakeout.

As shown in FIGS. 11A-11D, the vacuum gauge assembly 1100 can furtherinclude a cable 1105 between the electronics module 910 and the vacuumgauge 100, with the vacuum gauge 100 and electronics module 910displaced from each other. As shown in FIG. 11B, the cable 1105 has afirst end 1130 comprising a central anode connection pin, an anode guardinsulator spaced from and surrounding the anode connection pin, a guardring connection surrounding and spaced from the anode guard ringinsulator and an outer cylindrical insert having an interlock groove.Also shown in FIG. 11B, the cable 1105 has a first end 1130 thatincludes surfaces denoted with primes that imitate physical matingsurfaces of the vacuum gauge 100 shown in FIGS. 1A-1C, including guardring connection 102′, anode guard ring insulator 106′, anode connection110 a′, and the groove 1010′ that locks into interlock 940 describedabove. The first end 1130 also includes a wire connection 1140 inside ahousing 1135. The wire connection 1140 is connected to the outer shieldof the cable 1105, and serves to make the (grounded) connection to thecathode by connecting the u-shaped clip 1141, shown in FIG. 11C, arounda bolt on the outer flange 105 a (not shown). The housing 1135 is shownopen into two halves 1135 a and 1135 b in FIG. 11E, where retaining ring1136 that holds the physical mating surfaces in place is also shown. Asshown in FIG. 11D, the electronics module 910 can further include aninterlock 940 (shown in the open position in FIG. 11D) configured tolock to the first end 1130.

The cable 1105 also includes a second end 1150, shown in FIG. 11C, thatis configured to imitate physical mating surfaces, denoted with doubleprimes in FIG. 11C, of the electronics module 910 to mate to the vacuumgauge 100. As shown in FIG. 11C, the second end 1150 includes guard ringmating surface 102″, guard ring insulator mating surface 106″, and anodeconnection 110 a″ as well as wire 1140 and clip 1141 discussed above.

Methods of operating a cold cathode ionization vacuum gauge describedherein include setting a voltage potential difference to form anelectrical discharge between the anode electrode and the cathodeelectrode, measuring a discharge impedance between the anode electrodeand the cathode electrode, and deriving a pressure reading therefrom.

In one aspect, a method of operating a cold cathode ionization vacuumgauge includes switching the voltage potential difference between a highvoltage setting and a low voltage setting at a lower pressure than thatof a high voltage measurement anomaly and at a higher pressure than thatof a low voltage measurement anomaly. Measurement anomalies ordiscontinuities in the calibration curves for current and voltage as afunction of pressure in cold cathode ionization vacuum gauges are wellknown. See P. A. Redhead, Instabilities in crossed-field discharges atlow pressures, Vacuum vol. 38 pp. 901-908 (1988). These measurementanomalies are dependent on the specific geometry of the gauge, and occurover a limited pressure range at particular voltages. For example, asshown in FIG. 12, the cold cathode ionization vacuum gauges describedherein typically exhibit a discontinuity (a sudden jump in the measuredquantity (e.g., impedance or discharge current as described below)) atabout 5×10⁻⁶ Torr for a voltage potential difference of 2,000 V (i.e., alow voltage measurement anomaly), and a discontinuity at about 1×10⁻⁵Torr for a voltage potential difference of 3,500 V (i.e., a high voltagemeasurement anomaly).

The high voltage measurement anomaly and the low voltage measurementanomaly can be discharge current anomalies, or discharge impedanceanomalies. As shown in FIG. 13, a graph of discharge current as afunction of pressure shows that the discharge current becomes non-linearat pressures higher than about 1×10⁻⁵ Torr, asymptotically reaching amaximum discharge current. As shown in FIG. 4, this current limit is dueto a limiting resistor 410 in the discharge current measurement circuit,which sets an upper limit on the discharge current, thereby preventingexcessive sputtering of the cathode electrode. Consequently, as shown inFIG. 14, at pressures higher than about 1×10⁻⁵ Torr, the anode voltagedecreases. Therefore, in order to derive a pressure reading over alarger pressure range, the discharge impedance Z (Z=V/I_(D)) iscalculated as a function of pressure, because the discharge impedanceremains substantially linear over a wider pressure range, as shown inFIG. 15. See U.S. Pat. No. 4,967,157.

Turning back to FIG. 4, the limiting resistor 410 is placed between theanode electrode 110 and the high voltage power supply 430 (HVPS). Therole of the limiting resistor 410 is to put an upper limit to the amountof discharge current that can flow through the discharge volume 130 andto extend the lifetime of the vacuum gauge. As a result of the limitingresistor 410, the actual high voltage bias present at the anodeelectrode 110 and measured by voltmeter 420 is generally smaller thanthe voltage delivered by the HVPS 430. In fact, the anode voltagedecreases as the pressure increases, even though the output of the HVPS430 remains constant at all pressures. In the vacuum gauge describedherein, a 25 Megaohm (MΩ) limiting resistor 410 was selected to provideseveral advantages: 1. a safety limit to the amount of current the HVPScan deliver to an individual in case of accidental contact with internalHVPS components, 2. the choice of resistor moves pressure curvediscontinuities into the higher pressure range above 1×10⁻⁶ Torr, and 3.provides an upper limit for the discharge current of 125 μA. The coldcathode gauge controller 490 ensures that the output of the HVPS 430 isconstant over the entire pressure range while at the same timecontinuously measures the anode voltage V with voltmeter 420 anddischarge current I_(D) with ammeter 460 to calculate dischargeimpedance Z as a function of pressure.

In order to avoid discontinuities, it is important to avoid reaching thevoltages that lead to discontinuities at pressures that support thosediscontinuities. One possible solution to avoid discontinuities is tooperate the CCIVGs with low anode voltages and the smallest possiblecurrent limiting resistor, for example, by using a 2.0 kV High VoltageSupply and a 20 MΩ limiting resistor to make sure that the anode voltageremains below the discontinuity voltage throughout the entire range ofthe gauge. Operating the CCIVG with a low High Voltage supply, such thatthe anode voltage remains below the voltages that lead todiscontinuities, is a good approach, because the resulting current andimpedance curves do not have large discontinuities. However, thesensitivity of the gauge is lower at a low anode voltage potentialdifference, as a result of the reduced electric field inside theionization volume. For example, the cold cathode ionization vacuum gaugedescribed herein has a sensitivity as large as 12 A/Torr for a 5 kV highvoltage setting, but decreases to 1 A/Torr at 2 kV. The solutiondescribed herein is to operate the gauge at two different voltagesettings, a high voltage setting at low pressure, and a low voltagesetting at higher pressure. The high voltage setting can be in a rangeof about 3.5 kV to about 6 kV, and the low voltage setting can be in arange of about 2 kV to about 3 kV. Turning back to FIG. 12, the vacuumgauge controller begins operating the vacuum gauge with the output ofthe high voltage power supply (HVPS) at the high voltage setting, andautomatically shifts the output of the HVPS to the low voltage settingas the pressure crosses the HVPS crossover pressure. The vacuum gaugecontroller includes calibration curves to derive pressure at both thehigh voltage setting and the low voltage setting. Switching the voltagepotential difference between a high voltage setting and a low voltagesetting at a lower pressure than that of a high voltage measurementanomaly and at a higher pressure than that of a low voltage measurementanomaly avoids both the high voltage measurement anomaly (HOV:Discontinuity shown in FIG. 12) on the high pressure side of the HVPSCrossover Pressure level, and the low voltage measurement anomaly (LOV:Discontinuity shown in FIG. 12) on the low pressure side of the HVPSCrossover Pressure level, as shown in FIG. 12. Switching to severaldifferent voltage settings, and not just between a high voltage settingand a low voltage setting, could be implemented to avoid multiplediscontinuities.

In addition to avoiding discontinuities, the dual voltage mode ofoperation provides several additional advantages: 1. It provides theability to increase the anode voltage levels at low pressures withouthaving to worry about the effects of higher pressures. This is ideal forUHV operation where a gauge could operate at a very high voltage for UHVmeasurement and switches to a much lower voltage before thediscontinuity is reached. This provides high sensitivity at UHV, avoidsthe discontinuities, and provides protection of the gauge at higherpressures where a low voltage minimizes sputtering. Operation at highvoltage at UHV provides better sensitivity and also faster start times.2. It provides the ability to avoid discontinuities providing improvedaccuracy and repeatability. 3. It provides lower wear and tear at highpressures by minimizing sputtering.

Minimizing sputtering is important for the lifetime of a cold cathodeionization vacuum gauge, but there are circumstances when a higherdischarge current is tolerable during normal operation of the vacuumgauge. Another method of operating a cold cathode ionization vacuumgauge includes measuring a leakage electrical current between anelectrically conductive guard ring electrode interposed between thecathode electrode and the anode electrode about a base of the anodeelectrode, and triggering a gauge maintenance alarm if the pressurereading is less than an oscillatory discharge current pressure level andthe leakage electrical current is greater than a maximum allowableleakage current limit. The oscillatory discharge current pressure levelcan be about 5×10⁻⁶ Torr. The maximum allowable leakage current limitcan be about 1 μA. Turning back to FIG. 4, the two independent ammeters(460, 470) in the controller electronics 490 enable independentmeasurement of the plasma discharge current (I_(D)) by ammeter 460 andthe guard ring current (I_(GR)), which is the sum of the starter fieldemission (I_(FE)) current and feedthrough leakage current (I_(L)), bythe other ammeter 470. The controller 490 monitors the guard ringcurrent I_(GR) so that this current does not exceed a maximum allowablecurrent value, such as 1 μA. The starter field emission current I_(FE)is typically below 20 nA, and therefore as soon as any significantcontamination builds up on the feedthrough 103, the feedthrough leakagecurrent I_(L) is much greater than the starter field emission currentI_(FE), and therefore the feedthrough leakage current I_(L) isresponsible the majority of the guard ring current I_(GR). However, atpressures greater than, for example, about 5×10⁻⁶ Torr (the oscillatorydischarge current pressure level), the vacuum gauge can experienceoscillatory discharge currents that develop along the gauge axis, andalso reach the guard ring. As a result, the current measured by theguard ring ammeter 470 can be substantially larger than the 1 μA limit,such as in a range of about 1 to about 5 μA, but that is not a validreason to trigger a gauge maintenance alarm and stop operation of thegauge, because the high current level does not actually indicateexcessive contamination of the feedthrough 103.

Oscillatory discharge currents are not expected below a certainpressure, which is dependent on the particular design of the vacuumgauge. Therefore, if the maximum allowable leakage current limit isexceeded and the pressure is less than the oscillatory discharge currentpressure level, i.e., where oscillatory discharge currents are notexpected, then it is likely that the gauge has developed excessivefeedthrough leakage current and/or starter field emission current and agauge maintenance alarm is triggered. If, on the other hand, the maximumallowable leakage current limit is exceeded and the pressure is higherthan the oscillatory discharge current pressure level, then it is likelythat oscillatory discharge currents are responsible for the increase incurrent and no gauge maintenance alarm is triggered. The specificoscillatory discharge current pressure level depends on the gauge designand operational conditions. To check on the operational viability of thegauge, the gauge needs to be evacuated to a pressure below theoscillator discharge current pressure level and the feedthrough leakagecurrent needs to be measured and compared to the maximum allowableleakage current limit.

Yet another method of estimating the operational viability of a coldcathode ionization vacuum gauge includes measuring a discharge currentbetween the anode electrode and the cathode electrode, and deriving apressure reading therefrom, recording the discharge current as afunction of time, and integrating the discharge current over time toobtain a pressure dose for the vacuum gauge. Pressure dose as a measureof the effective lifetime of cold cathode ionization vacuum gauges wasintroduced by Wilfert and Schindler in 2004. See St. Wilfert and N.Schindler, Applied Physics A vol. 78, pp. 993-666 and 691-694 (2004).Wilfert and Schindler integrated the measured pressure over time toobtain the pressure dose PD(T) according to the expression

PD(T)=∫₀ ^(T) P(t)dt   (2)

where P(t) is the pressure measured at time t, and T is the elapsed time(hours) since the beginning of measurements. They concluded that a coldcathode ionization vacuum gauge operated in a typical residual gasenvironment has an accuracy degradation in a range of about 10% to about30% after a pressure dose of 1.1 mbar*h.

Integrating the measured pressure is not suitable at high pressures forcold cathode ionization vacuum gauges that have limiting resistors,because, as discussed above, for such vacuum gauges the pressure is notdirectly related to the discharge current, and it is more likely thatthe discharge current is responsible for the accuracy degradation of thegauge due to sputtering of the cathode. The pressure dose expressionconverted to discharge current is

PD(T)=∫₀ ^(T) I _(D)(t)dt   (3)

where I_(D)(t) is the discharge current at time t, and T is the elapsedtime since the beginning of measurements. The Wilfert and Schindler 1.1mbar*h converts to an integrated charge of about 3,600 Coulombs.

The method can further include recording and integrating a gas factor asa function of time, to account for gas species dependent sputteringrates, using the expression

PD(T)=∫₀ ^(T) G(t)I _(D)(t)dt   (4)

where G(t) is a gas species factor at time t (default is equal to 1 fornitrogen), I_(D)(t) is the discharge current at time t, and T is theelapsed time since the beginning of measurements.

The method can further include recording and integrating an ion energyfactor as a function of time, because the ion energy is likely to affectthe sputtering rate of the cathode. The integration over time of thesputtering rate provides a closer approximation of the pressure dose.The sputtering rate depends on the discharge current (i.e., the numberof ions hitting the cathode per unit time) and it is weighted by the gasspecies and the energy of the ions reaching the cathode. An energyfactor is therefore introduced to account for the fact that the energyof the ions depends on the pressure (i.e., the anode voltage changeswith pressure, due to the limiting resistor discussed above). Theexpression including the energy factor is

PD(T)=∫₀ ^(T) E(t)G(t)I _(D)(t)dt   (5)

where E(t) is the energy factor at time t, G(t) is a gas species factorat time t (default is equal to 1 for nitrogen), I_(D)(t) is thedischarge current at time t, and T is the elapsed time since thebeginning of measurements. The energy factor is higher at lowerpressures and lower at higher pressures where the anode voltagedecreases relative to the high voltage power supply output (see FIG.14).

Additionally, the method can include determining a remaining servicelife for the vacuum gauge based on the pressure dose. The remainingservice life is calculated using

$\begin{matrix}{{{Life}(\%)} = {\frac{{{EF}*{PD}_{\max}} - {{PD}(T)}}{{EF}*{PD}_{\max}}*100\%}} & (6)\end{matrix}$

where EF is an environmental factor (default=1), and PD_(max) is themaximum acceptable dose (default is 3,600 Coulombs as discussed above).The environmental factor can be adjustable, enabling adjustment in therate at which the remaining service life decreases as the pressure doseincreases. An environmental factor greater than 1 allows a largermaximum pressure dose, increasing the service life of the vacuum gauge.Vacuum gauge that operate in more benign environments or ones that cantolerate larger changes in sensitivity before replacement of the vacuumgauge can use an EF>1. On the other hand, vacuum gauges that operate inharsher environments (e.g., including exposure to corrosive gases, suchas chlorine) or ones that need to provide higher accuracy can use anEF<1, leading to more frequent replacement of the vacuum gauge. Theremaining service life diminishes from 100% to 0% as the pressure doseprogresses from 0 Coulombs to (PD_(max)*EF).

The relevant teachings of all patents, published applications andreferences cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A cold cathode ionization vacuum gauge assemblycomprising an ion gauge, the ion gauge comprising: an extended anodeelectrode; a cathode electrode surrounding the anode electrode along itslength, one end of the cathode electrode being connected to a flange;and a magnetic coupler connected to the flange; and a magnet assemblyconfigured to be slidably mounted over the cathode and magneticallycoupled to the magnetic coupler.
 2. The cold cathode ionization vacuumgauge assembly of claim 1, further including an electronics moduleconfigured to be directly coupled to the vacuum gauge with an interfacecomplementary to the vacuum gauge, the module housing electronicsadapted to operate and read the vacuum gauge.
 3. The cold cathodeionization vacuum gauge assembly of claim 2, further including aninterlock on the electronics module configured to lock the electronicsmodule to the ion vacuum gauge.
 4. The cold cathode ionization vacuumgauge assembly of claim 2, further including a magnet on a front face ofthe electronics module adapted to hold the vacuum gauge in place untilthe interlock is engaged.
 5. A cold cathode ionization vacuum gaugeassembly comprising: a cold cathode ionization vacuum gauge comprising:an extended anode electrode; and a cathode electrode surrounding theanode electrode along its length; an electronics module configured to bedirectly coupled to the vacuum gauge with an interface complementary tothe vacuum gauge, the module housing electronics adapted to operate andread the vacuum gauge; and a cable between the electronics module andthe vacuum gauge with the vacuum gauge and electronics module displacedfrom each other, the cable having a first end and a second end, thefirst end being configured to imitate physical mating surfaces of thevacuum gauge to mate to the electronics module, and the second end beingconfigured to imitate physical mating surfaces of the electronics moduleto mate to the vacuum gauge.
 6. The cold cathode ionization vacuum gaugeassembly of claim 5, further including an interlock on the electronicsmodule configured to detect the presence of the vacuum gauge or thefirst end of the cable.
 7. A cable for connecting an electronics moduleto a cold cathode ionization vacuum gauge, with the vacuum gauge andelectronics module displaced from each other, the cable having a firstend and a second end, the first end being configured to imitate physicalmating surfaces of the vacuum gauge to mate to the electronics module,and the second end being configured to imitate physical mating surfacesof the electronics module to mate to the vacuum gauge.
 8. The cable ofclaim 7, wherein the first end comprises a central anode connection pin,an anode guard ring insulator spaced from and surrounding the anodeconnection pin and a guard ring connection surrounding and spaced fromthe anode guard ring insulator.
 9. The cable of claim 8, wherein thefirst end further comprises an outer cylindrical insert having aninterlock groove.
 10. The cable of claim 7, wherein the second endcomprises a central anode connection sleeve, a guard ring insulatormating sleeve and a guard ring mating sleeve.
 11. A method of operatinga cold cathode ionization vacuum gauge comprising: setting a voltagepotential difference to form an electrical discharge between the anodeelectrode and the cathode electrode; measuring a discharge impedancebetween the anode electrode and the cathode electrode; and switching thevoltage potential difference between a high voltage setting and a lowvoltage setting at a lower pressure than that of a high voltagemeasurement anomaly and at a higher pressure than that of a low voltagemeasurement anomaly.
 12. The method of claim 11, wherein the highvoltage measurement anomaly and the low voltage measurement anomaly aredischarge current anomalies.
 13. A method of operating a cold cathodeionization vacuum gauge comprising: setting a voltage potentialdifference to form an electrical discharge between the anode electrodeand the cathode electrode; measuring a discharge impedance between theanode electrode and the cathode electrode, and deriving a pressurereading therefrom; measuring a leakage electrical current between anelectrically conductive guard ring electrode interposed between thecathode electrode and the anode electrode about a base of the anodeelectrode; and triggering a gauge maintenance alarm if the pressurereading is less than an oscillatory discharge current pressure level andthe leakage electrical current is greater than a maximum allowableleakage current limit.
 14. The method of claim 13, wherein theoscillatory discharge current pressure level is about 5×10⁻⁶ Torr. 15.The method of claim 13, wherein the maximum allowable leakage currentlimit is about 1 μA.
 16. A method of operating a cold cathode ionizationvacuum gauge comprising: setting a voltage potential difference to forman electrical discharge between the anode electrode and the cathodeelectrode; measuring a discharge current between the anode electrode andthe cathode electrode, and deriving a pressure reading therefrom;recording the discharge current as a function of time; and integratingthe discharge current over time to obtain a pressure dose for the vacuumgauge.
 17. The method of claim 16, further including recording andintegrating a gas factor as a function of time.
 18. The method of claim16, further including recording and integrating an ion energy factor asa function of time.
 19. The method of claim 16, further includingdetermining a remaining service life for the vacuum gauge based on thepressure dose.